1. Field of the Invention:
The invention relates generally to spinal implants for use in intervertebral disc replacement; and more specifically relates to articulating implants that fuse to adjacent vertebra by bone ingrowth, thus restoring proper intervertebral spacing, and eliminating nerve root and/or spinal cord compression, while preserving spinal flexibility.
2. Description of Related Art
The spinal column is formed from a number of vertebra, which in their normal state are separated from one another by cartilaginous intervertebral discs. These discs form a cushion between adjacent vertebra, resisting compression along the support axis of the spinal column, but permitting limited movement between the vertebra to provide the characteristic flexibility of the healthy spine. Injury, disease or other degenerative disorders may cause one or more intervertebral discs to shrink, collapse, deteriorate or become displaced, herniated, or otherwise damaged. This can lead to compression of adjacent nerve root(s) or the spinal cord causing chronic and often disabling pain, and in advanced circumstances, irreversible paralysis of upper and/or lower limbs.
A number of devices and methods have been suggested for the replacement of damaged or dislocated intervertebral discs. One common approach is to permanently stabilize or “fuse” the adjacent vertebra to maintain the proper intervertebral spacing and eliminate relative movement between the vertebra. Various methods of vertebral stabilization have been developed, for example, autogenous grafts of dowel-shaped sections of bone have been implanted between the vertebra to cause bone growth across the intervertebral space, thereby fusing the adjacent vertebra into one bone mass. This procedure disadvantageously requires the harvest of donor bone for the graft from other parts of the patient's body, typically requiring a separate surgical procedure and resultant increases in complications and expense. An alternative source is cadaver bone, with potential complications of transmissible diseases, impaired graft incorporation, collapse or displacement. A further development to this method of vertebral stabilization involves the implantation of a perforated cylindrical bone basket between adjacent vertebra. Bone fragments produced in preparing the vertebra for the implantation are inserted into the bone basket to promote bone growth into, through, and around the basket.
Vertebral stabilization by fusion of adjacent vertebra has proven successful in permanently preserving intervertebral spacing, but has been found to present a number of disadvantages. Fusion of adjacent vertebra necessarily eliminates a portion of the spine's normal range of motion, thereby reducing the subject's spinal flexibility. Additionally, long-term investigations after fusion of the vertebra have revealed deterioration in the next adjacent segments, ranging from increasing osteophyte formation, to collapse of the intervertebral disc, implicating progressive degeneration of the afflicted motion segment, resulting in hyper- and/or hypomobility of the segment. It is believed that fusion results in nonphysiological bio-mechanical stress of the adjacent segment. In addition to the nonphysiological bio-mechanical stress, there is an increase in the intradiscal pressure (PID) on the adjacent intervertebral disc. Changes in the PID may impair the nutrition of the intervertebral disc, which can lead to increased degeneration and pain in any plane of motion and/or compression of the spinal cord on spinal nerves.
It has also been proposed to replace an injured intervertebral disc with a prosthesis which is “jointed” to permit relative movement between vertebra. Previously known devices of this type generally have been found to suffer from inadequate attachment between the prosthesis and the vertebra. The intended movement between the components of previously known jointed prostheses can cause relative motion between the prosthesis and the adjacent bone surface(s). Because such motion would disrupt bone ingrowth, jointed prostheses have generally been considered incompatible with attachment by bone ingrowth. In addition, because the joint elements of these devices typically must occupy a substantial vertical extent in order to achieve the desired range of motion, and yet must fit within the intervertebral space, attachment of such devices generally has been by use of flat plates or surfaces provided on either side of the joint elements as points of fixation to the vertebra. This attachment may be accomplished by compressive or friction fits, spiked projections, screws or pins, complemented in some instances with tissue ingrowth into porous surfaces. These mechanisms of attachment, however, may lack the degree and strength of fixation desired. Moreover, several such devices have used attachment flanges which extend beyond the surfaces of the vertebra to which the device is attached. This has been found undesirable, as the extending flanges may interfere with or injure adjacent tissue; for example, it has been reported that flanges extending into immediately adjacent delicate esophageal areas may interfere with swallowing and speech, or cause perforation and potentially fatal infection. An additional drawback to the use of screw and pin connections is the potential for such connectors to dislodge and cause injury. Examples of such prosthetic implants are disclosed in the following: U.S. Pat. No. 4,759,769, to Hedman et al., U.S. Pat. No. 4,946,378, to Hirayama et al., U.S. Pat. No. 4,997,432, to Keller, U.S. Pat. No. 5,002,576, to Fuhrmann et al., U.S. Pat. No. 5,236,460, to Barber, U.S. Pat. No. 5,258,031, to Salib et al., U.S. Pat. No. 5,306,308, to Gross, et al., U.S. Pat. No. 5,401,269, to Buttner-Janz, et al., U.S. Pat. No. 5,425,773, to Boyd, et al. and U.S. Pat. No. 5,782,832, to Larsen, et al.
Thus, it can be seen that a need yet exists for a spinal implant effective in permanently maintaining intervertebral spacing to prevent nerve or spinal cord compression, while preserving as much of the natural range of motion between the affected vertebra as possible. A need further exists for such a device which is capable of forming a permanent, strong attachment to the vertebra, yet does not protrude beyond the external surfaces to which it is attached. Still another need exists for a method of replacing a damaged or displaced disc, maintaining intervertebral spacing to prevent nerve and spinal cord compression, while preserving the natural relative motion between the vertebra. It is to the provision of devices and methods meeting these and other needs that the present invention is primarily directed.